Multipole technology is used generally for charged particle optics which includes separating, focusing, or collimating "charged particles" (i.e., ions, electrons, etc.). A primary application of multipole technology is quadrupole mass filters. Mass filters are tools for analyzing the chemical composition of matter by using electric fields to separate charged particles. Quadrupole mass filters have four parallel elongated poles (i.e., electrodes) and opposing parallel poles are electrically connected. The poles have a cross-section that closely approximates hyperbolic arcs in respective quadrants about a common origin.
A radio-frequency power amplifier (RFPA) drives both pairs of poles. A selected radio frequency (RF) signal summed with a positive direct current (DC) potential drives one set of poles. An RF signal, 180.degree. out of phase with that applied to the first pair, summed with a negative DC potential drives the other pair of poles.
The RF field dominates the motion of relatively light charged particles, ejecting them from the functional center region of the quadrupole filter. The DC field dominates the relatively heavy charged particles and causes poles to attract and adsorb charged particles of opposite conductivity. Charged particles of an appropriate intermediate weight can traverse a generally longitudinal trajectory through the center of the quadrupole due to offsetting RF and DC effects.
By properly setting the RF and DC components of the mass selection field inside the quadrupole, the quadrupole can select for detection and measurement any mass within the operating range of the unit. Alternatively, a quadrupole can function as a high pass filter. The DC component equals zero and RF amplitude determines the low mass transmission limit.
The theoretically ideal cross section for the four poles of a quadrupole mass filter is four hyperbolic curves extending in their respective quadrants to infinity. Generally, the quadrupole mass filter approximates only the portion of the hyperbolic arcs near their origins. They approximate the arcs with solid metal rods (e.g., molybdenum or stainless steel) that have been ground to a desired shape. The quadrupole mass filters maintain the desired relative arrangement of the four ground rods by a harness of ceramic or other rigid, non-conductive material.
However, there are several disadvantages to this four rod implementation of a quadrupole filter: expense, weight, bulk, and vulnerability to misalignment. For example, grinding identical hyperbolic surfaces on four several-inch long molybdenum rods is costly both in terms of time and materials. Furthermore, only the hyperbolic surface is electrically useful. The bulk of the rod serves only limited functions such as providing rigidity. If an internal or external force jolts the four rods in the ceramic harnesses, misalignment can easily occur. Furthermore, this misalignment may be undetectable by an unaided eye, and yet adversely affect the quality of performance.
U.S. Pat. No. 3,328,146 Method of Producing An Analyzer Electrode System For Mass Spectrometers, issued to Hanlein and assigned to Siemens-Schuckertwere Aktiengesellschaft and U.S. Pat. No. 4,885,500 Quartz Quadrupole For Mass Filter, issued to Hansen et al. and assigned to Hewlett-Packard Company describe quadrupole mass filters made from a glass quadrupole tube and thin strips of metal. The glass quadrupole tube has a cross-section of four interconnected truncated hyperbolas, semicircles, etc. that provide a substrate for the four poles of the quadrupole. Thin strips of metal conform to these four pole substrates and create four poles with a hyperbolic cross-section that produces an electric field with a hyperbolic shape.
Glass quadrupole mass filters have the advantage of eliminating the primary problems of the four rod quadrupple mass filters: weight, bulk, cost of manufacture, and vulnerability to misalignment. Glass quadrupole mass filters have the advantage of greatly reduced weight and bulk due to the substitution of glass and thin strips of metal for the refractory metal rods. Glass greatly reduces manufacturing costs since it is inexpensive and easily transforms into the desired quadrupole shape of a mandrel. This reduces the costs and time involved in grinding refractory metal rods from four rods per mass filter to one mandrel that forms many mass filters. Additionally, glass usually is less susceptible to small inelastic deformations than refractory metals, so glass quadrupoles produce valid measurements unless the glass breaks.
Quadrupole mass filters separate charged particles whose mass/charge ratio differs by approximately 1 AMU. To accomplish this, the poles must produce precisely-shaped hyperbolic electric fields. Additionally, electric fields produced by two adjacent poles should be out of phase by 180.degree., but otherwise have an identical shape and magnitude. If the poles fail to produce electric fields meeting these specifications, the quadrupole output may be less than optimal and the quadrupole may have impaired resolution. To produce electric fields that meet the specifications listed above, the poles must be thick enough that the resistance down the length of the poles is very low and the poles must precisely conform to the glass substrate of the quadrupole so that they have a hyperbolic cross-section.
U.S. Pat. No. 3,328,146 discloses forming a single metal metallized or mirrored surface on the hyperbolic glass surfaces by vaporizing or cathode sputtering gold on them. These gold poles may have several problems; poor adhesion, relatively high resistance resulting from a thin coating of gold, nonuniform thickness, and they may be difficult to make consistently in a manufacturing environment. Poor adhesion partially results from the weak bonds that pure gold forms with glass. Gold oxides can be created which would form strong bonds but it would convert back to pure gold at the high temperatures typical of an operational quadrupole mass filter. This pure gold would peel off the quadrupole. A relatively high resistance would produce a voltage drop down the approximately four to twelve inch length of the pole and would impair the ability of the mass filter to separate charged particles. Another problem with the sputtered gold pole would be the nonuniform thickness of the pole that would distort the shape of the electric field and impair the ability of the quadrupole mass filter to separate charged particles.
U.S. Pat. No. 4,885,500 teaches creating poles by positioning thin strips of silver having an adhesive backing ("silver tape") to the hyperbolic contours of the inner surface of the glass substrate. The silver tape must conform uniformly to the hyperbolic contours of the glass substrate to produce poles with a hyperbolic cross section and to produce electric fields with the desired hyperbolic shape. The primary disadvantages of previously-existing glass quadrupole mass filters include contamination of the silver tape by subsequent processing and the difficulty of manufacturing them in a highly controlled manner.